The one exception to this is if you have nested loops not because you're going over your data multiple times, but simply because that's the most natural way to

The one exception to this is if you have nested loops not because you're going over your data multiple times, but simply because that's the most natural way to iterate over your data given how it's organized. If you have eg. a 2D array, then doubly-nested loops are usually the most straightforward way to iterate over the elements and aren't necessarily a red flag—many of the useful things you can do to a 2D array require touching each element at least once. But if you have triply-nested loops working on a 2D array rather than a 3D array, it's worth a second look.